1. Field of the Invention
This invention relates to a magnetic core composed of an amorphous metallic alloy and adapted for electrical choke applications such as power factor correction (PFC) wherein a high DC bias current is applied.
2. Description of the Prior Art
An electrical choke is a DC energy storage inductor. For a toroidal shaped inductor the stored energy is W=1/2[(B.sup.2 A.sub.c l.sub.m)/(2.mu..sub.O .mu..sub.r)], where B is the magnetic flux density, A.sub.c the effective magnetic area of the core, .sup.1 m the mean magnetic path length, and .mu..sub.O the permeability of the free space and .mu..sub.r , the relative permeability in the material.
By introducing a small air gap in the toroid, the magnetic flux in the air gap remains the same as in the ferromagnetic core material. However, since the permeability of the air (.mu..about.1) is significantly lower than in the typical ferromagnetic material (.mu..about.several thousand) the magnetic field strength(H) in the gap becomes much higher than in the rest of the core (H=B/.mu.). The energy stored per unit volume in the magnetic field is W=1/2(BH), therefore we can assume that it is primarily concentrated in the air gap. In other words, the energy storage capacity of the core is enhanced by the introduction of the gap. The gap can be discrete or distributed.
A distributed gap can be introduced by using ferromagnetic powder held together with nonmagnetic binder or by partially crystallizing an amorphous alloy. In the second case ferromagnetic crystalline phases separate and are surrounded by nonmagnetic matrix. This partial crystallization method is achieved by subjecting an amorphous metallic alloy to a heat treatment. Specifically, there is provided in accordance with that method a unique correlation between the degree of crystallization and the permeability values. In order to achieve permeability in the range of 100 to 400, crystallization is required of the order of 10% to 25% of the volume. The appropriate combination of annealing time and temperature conditions are selected based on the crystallization temperature and or the chemical composition of the amorphous metallic alloy. By increasing the degree of crystallization the permeability of the core is reduced. The reduction in the permeability results in increased ability of the core to sustain DC bias fields and increased core losses.
A discrete gap is introduced by cutting the magnetic core and inserting a nonmagnetic spacer. The size of the gap is determined by the thickness of the spacer. Typically, by increasing the size of the discrete gap, the effective permeability is reduced and the ability of the core to sustain DC bias fields is increased. However, for DC bias excitation fields of 100 Oe and higher, gaps of the order of 5-10 mm are required. These large gaps reduce the permeability to very low levels (10-50) and the core losses increase, due to increased leakage flux in the gap.
For power factor correction applications in power equipment and devices there is a need for a small size electrical choke with low permeability(50-300), low core losses, high saturation magnetization and which can sustain high DC bias magnetic fields.